Reactive sulfur species (RSS) are a family of sulfur-containing molecules found in biological systems. These molecules include thiols, S-modified cysteine adducts such as S-nitrosothiols and sulfenic acids, hydrogen sulfide, persulfide, polysulfide, as well as inorganic sulfur derivatives. RSS have attracted increasing attention in biomedical research because these molecules show a variety of physiological functions (M. C. H. Gruhlke and A. J. Slusarenko, Plant Physiol. Biochem., 2012, 59, 98).
For example, hydrogen sulfide (H2S) has been recently recognized as a new gaseous transmitter. The production of endogenous and exogenous administration of H2S have been demonstrated to exert protective effects in many pathologies (L. Li, P. Rose and P. K. Moore, Annu. Rev. Pharmacol. Toxicol., 2011, 51, 169). Sulfane sulfur compounds are another type of important RSS (E. G. Mueller, Nat. Chem. Biol., 2006, 2, 185). Sulfane sulfur refers to sulfur atom with six valence electrons but no charge (represented as S0). Biologically important sulfane sulfur compounds include persulfides (R—S—SH), hydrogen persulfide (H2S2), polysulfides (R—S—Sn—S—R), and protein-bound elemental sulfur (S8). Sulfane sulfur has unique reactivity to attach reversibly to other sulfur atoms and exhibit regulatory effects in diverse biological systems. These functions include post-transcriptional modification of transfer RNA, synthesis of sulfur-containing cofactors and vitamins, activation or inhibition of enzymes. H2S and sulfane sulfur always coexist and recent work suggests that sulfane sulfur species, derived from H2S, may be the actual signaling molecules (J. I. Toohey, Anal. Biochem. 2011, 413, 1).
Despite the rising interest in sulfane sulfur research, many fundamental questions regarding their production and mechanism of actions remain to be clarified. It is important, therefore, to understand the chemistry and properties of sulfane sulfur species. Accurate and reliable measurement of sulfane sulfur concentrations in biological samples is needed and can provide useful information to understand their functions. Currently the only method for sulfane sulfur detection is based on the reaction with cyanide ion to form thiocyanate, which can then be measured as ferric thiocyanate (J. L. Wood, Methods Enzymol., 1987, 143, 25). However, this method requires post-mortem processing and destruction of tissues or cell lysates. Therefore it cannot be applied in real-time detection in biological samples. Fluorescence assays could be very useful in this field due to the high sensitivity and convenience. Although much progress have been made in the development of fluorescent probes for H2S (Patent application number US 20120329085 A1) and biological thiols ((a) X. Chen, Y. Zhou, X. Peng and J. Yoon, Chem. Soc. Rev., 2010, 39, 2120; (b) J. Lu and H. Ma, Chin. Sci. Bull. (Chin. Ver.), 2012, 57, 1462; (c) H. Peng, W. Chen, Y. Cheng, L. Hakuna, R. Strongin and B. Wang, Sensors, 2012, 12, 15907; (d) Z. Guo, S, Nam, S. Park and J. Yoon, Chem. Sci., 2012, 3, 2760; (e) K. Xu, M. Qiang, W. Gao, R. Su, N. Li, Y. Gao, Y. Xie, F. Kong and B. Tang, Chem. Sci., 2013, 4, 1079) fluorescent probes for sulfane sulfur detection are still unavailable. Here we report a first reaction-based fluorescent turn-on strategy for the detection of sulfane sulfurs.